Global data loaded if symbols aren't resolved

[Marcus Clyne]

Ian Lance Taylor wrote:
> Marcus Clyne <maccaday@gmail.com> writes:
>
>   
>>>> If a shared object is loaded using dlopen etc and the symbols in its
>>>> object aren't used (specifically they are defined elsewhere already
>>>> inside that process), is the data in the symbols automatically loaded
>>>> into memory or is it skipped (assuming there are no other programs
>>>> using those symbols on the system) and the memory available for other
>>>> purposes?
>>>>     
>>>>         
>>> If you open a shared library with dlopen, then that library is loaded
>>> into memory.  This will use up virtual address space.  If you don't use
>>> the library for anything, then the pages will never be loaded into RAM.
>>>   
>>>       
>> When the object loaded initially with dlopen(), is there anything
>> that's transferred to RAM (e.g. the page where the symbol names are),
>> or are they only copied to the swap space directly?  If that's the
>> case, are they then loaded into RAM when dlsym() is called?
>>     
>
> Most of a shared library is read-only and shared.  Those pages are
> simply mapped into your virtual address space--no copying occurs.  In a
> typical shared library, a few pages must be modified by the dynamic
> linker when the library is loaded: those pages will be copied into RAM
> and modified, and if not referenced will eventually be swapped out.
>
>   
Would pages that used symbols from other libraries be included in the 
pages that are copied and modified?  If that's the case, what about 
pages that have calls to symbols in the standard c library.  Are they 
also copied/modified?  What about global variables inside the shared 
object?  Presumably they are copied, no?

Thanks,

Marcus.